(1) Field of the Invention
The present invention relates generally to a system and method for automatically controlling a vehicle speed to a desired cruise speed.
(2) Background of the Art
A Japanese Utility Model Application First Publication Showa 60-3123 published on Jan. 11, 1985 exemplifies an automotive vehicle cruising speed controlling system and method.
A previously proposed vehicle automatic cruise control system of the Japanese Utility Model 60-3123 type is shown in FIG. 3. As shown in FIG. 3, the prior art system includes a target vehicle speed setting block having a set switch which is turned on and off produces a target speed signal V.sub.s of the vehicle. A control unit receives the target vehicle speed signal V.sub.s and an actual vehicle speed V derived from an integrator 10. After a predetermined arithmetic operation processing, a power supply pulse T.sub.p is outputted. An opening angle .theta. of a throttle valve is changed in response to the power supply pulse T.sub.p outputted from the control unit. A torque T.sub.E corresponding to the opening angle .theta. of the throttle valve is outputted to an engine transmission. The transmission converts the torque and outputs a wheel torque T.sub.w. A wheel force F.sub.w is outputted as F.sub.w =T.sub.w /R from a wheel portion. It is noted that R denotes an effective tire radius.
A vehicle running resistance D is detected by a vehicle running resistance detecting means. The vehicle running resistance includes, e.g., rolling resistance of a tire wheel, air resistance, gradient resistance, and so forth. The running resistance D is subtracted from the wheel force F.sub.w by means of a step S to determine a propelling force of the vehicle. A vehicle weight portion determines a vehicle acceleration V' with a total vehicle weight set as W. That is to say, V'=(F.sub.w -D)/W. The acceleration V' is integrated by means of an integrator 10 to derive the actual vehicle speed V. The actual vehicle speed V is inputted to the control unit to constitute a closed loop.
In the above-described previously proposed system for controlling a vehicle speed to the desired cruise speed, the control unit receives the target vehicle speed V.sub.s and actual vehicle speed V and calculates the desired power supply pulse T.sub.p. The power supply pulse T.sub.p is inputted into a throttle actuator to determine the opening angle of the throttle valve. The torque T.sub.E is transmitted to the transmission. The wheel torque T.sub.W is transmitted from the transmission to the wheel portion. The vehicle propelling force is derived through an arithmetic operation of the running resistance D and vehicle weight. The vehicle acceleration V' is derived at the vehicle weight portion. The acceleration V' is converted into the actual vehicle speed V by means of the integrator and is again inputted to the control unit. Hence, the opening angle of the throttle valve is properly adjusted by deriving the difference between the target vehicle speed V.sub.s and actual vehicle speed V.
However, there are problems in the above-described automatic cruise speed controlling system and method.
The control method executed by the control unit is divided into a steady state mode in which the actual vehicle speed V coincides with the target vehicle speed V.sub.s and in a transient mode in which the vehicle target speed V.sub.s which is changed transiently (increased or decreased) during the acceleration or deceleration of the vehicle is matched with the actual vehicle speed V.
At a time when the mode is switched, the opening angle .theta. of the throttle valve needs to be largely changed. Therefore, the switching between the steady state mode and transient state mode is not smoothly carried out due to variations in the characteristics of the throttle actuator and accelerator link.
The above-described switching is constituted by two system operations: (1) in order to accelerate or decelerate the vehicle, the mode is exchanged from the steady state operation mode to the transient operation mode; or (2) the mode is changed from the transient state operation mode to the steady state operation mode.
In the case of (1), the opening angle .theta. of the throttle valve is needed to increase step-wise toward (.theta.+.theta..sub.1). In the case of (2), the opening angle .theta. of the throttle valve is needed to be reduced step-wise toward (.theta.-.theta..sub.2). A pneumatic actuator constituted by bidirectional valves of a first valve (TIV) which actuates the throttle valve in the open direction and of a second valve (TDV) which actuates the throttle valve in the closed direction. In the above-described case (1), the power supply pulse T.sub.p1 which is previously set for the first valve and which compensates for the change in the opening angle .theta..sub.1 or of the throttle valve is needed to be outputted. Therefore, in a case where a deviation or a deterioration of an initial characteristic in the accelerator link or throttle actuator due to aging effect, for example, is present, the opening angle of the throttle valve is not smoothly adjusted by a change in the opening angle .theta..sub.1 or .theta..sub.2 outputted from the throttle actuator. Therefore, a predetermined acceleration cannot be achieved in the above-described case (1). The vehicle speed is changed due to unnecessary variation in the opening angle of the throttle valve in the case (2).
Consequently, the previously proposed automatic cruise speed controlling system can give an unpleasant feeling to a vehicle passenger(s) and/or driver.